1. Field of the Invention
This invention relates to a transfer control system of an information read beam for transferring an information read beam applied from the same application direction from one recording side to another of an information recording medium having a plurality of laminated recording sides.
2. Description of the Related Art
For example, a two-layer disc of a DVD (digital versatile disc) is known as the above-mentioned information recording medium. As shown in FIG. 1, the two-layer disc has a structure wherein a recording side of a substrate L1 coated with a total reflection film of aluminum (Al) and a recording side of a substrate L0 coated with a semi-transparent film formed of gold (Au), a dielectric substance, etc., providing a reflection factor of about 30% are laminated with a ultraviolet-setting resin material having high light transmittance such as a photopolymer resin with the recording sides facing each other. An information read beam is applied from one side (the substrate L0 side in FIG. 1). Information on each recording side can be read by forcibly transferring the focus position of the applied information read beam from the substrate L0 to the substrate L1 or from L1 to L0 (focus jumping).
A specific focus jumping method is as follows: To a focus actuator for driving an object lens in a focus direction, a kick signal at a predetermined level, which is a transfer signal for forcibly driving the object lens in acceleration, is supplied only for a predetermined time, and then a brake signal at a predetermined level, which is a transfer signal for forcibly driving the object lens in deceleration, is supplied for a predetermined time. The predetermined levels and the predetermined times are determined based on the sensitivity of the actuator (the acceleration produced when a unit drive current is supplied) and the read beam transfer distance.
For example, with a reader for reading digital information from a recording medium on which digital information is recorded such as a DVD or a CD (compact disc), the focus position of an object lens is set so that the spot form of a read beam on the disc becomes elliptic rather than circular to decrease the jitter component of a read signal. More particularly, as shown in FIG. 1, the focus position of the object lens is set so that the long axis direction of an elliptic spot 12 becomes perpendicular to the tangent direction of a track formed by pits 11 (the short axis direction of the elliptic spot becomes parallel to the tangent direction of the track). That is, in such focus servo control for the recording medium, a predetermined DC level is piggybacked as an offset level on a focus error signal indicating the shift amount from the focus position of the information read beam and servo control is performed based on the error signal with the offset level piggybacked thereon, thereby aggressively offsetting as much as the distance corresponding to the offset level from the focus position for changing the spot form of the read beam on the disc from circular to elliptic.
In the initialization operation before information is read, while the offset level to be piggybacked is swung in a predetermined range, for example, a read signal error rate is measured and the level at which the error rate becomes the minimum is set as the predetermined offset level mentioned above.
By the way, the optimum offset level is set separately for each of the two recording sides of a two-layer disc. Thus, if the above-described spot form shaping is performed on the two-layer disc, the read beam transfer distance by the above-mentioned focus jumping varies from one disk to another in response to the offset amount set for each recording side. Therefore, the amplitude levels and durations (pulse widths) of the kick signal and the brake signal of the transfer signals must be set for each disc. An S-letter waveform of a focus error signal appearing as the read beam is transferred is used to indicate the supply timing of the transfer signal (brake signal) for forcibly driving the object lens in deceleration by comparing the amplitude level of the S-letter waveform with a predetermined threshold value. As the above-mentioned offset is piggybacked, the amplitude level of the appearing S-letter waveform is offset in response to the read beam transfer direction (from one recording side to the other or from the other recording side to one), and the threshold value must be set in response to the transfer direction. Also, if the reflection factor varies from one recording side to another, the amplitude level of the appearing S-letter waveform changes with the recording side to which the read beam is transferred; the brake signal amplitude level and duration must be set each time and the setting job is intricate.
Since an offset is thus piggybacked to shape the read beam spot form, it becomes intricate to set the transfer signals involved in the focus jumping.